Plasma generator for a highly ionized electrical plasma



p 1963 J. M. WILCOX ETAL 3,104,345 PLASMA GENERATOR FOR-A HIGHLY IONIZEDELECTRICAL PLASMA 2 Sheets -Sheet 1 Filed Dec. 7, 1961 momDOm w 0mobwzmo M35 km\ 58;;

INVENTORS JOHN M. W/LCOX WILLIAM R. BAKER A TTOHNE Y Sept. 17, 1963 J.M. WILCOX ETAL PLASMA GENERATOR FOR A HIGHLY IONIZED ELECTRICAL PLASMA 2Sheets Sheet 2 Filed Dec. '7, 1961 lgnifron 32 INVENTORS JOHN M. W/LCOXBY WILLIAM R. BAKER A TTORNE Y U i d States Patent 3,104,345 PLASMAGENERATOR FOR A HIGHLY IGNEZED I ELECTRHIAL PLASMA John M. Wilcox,Berkeley, and William R. Baker, Orinda, Calif., assignors to the UnitedStates of America as represented by the United States Atomic EnergyCommission Filed Dec. 7, 1961, Ser. No. 157,863 Claims; (Cl. 315-111)Yreleasing contaminants which may prevent many desired nuclearinteractions from occuring. To avoid this, in many devices the plasma isproduced in one location and then moved to another location for heatingso that no interfering electrodes will be present. However, theadditional apparatus necessary for moving the plasma adds to thecomplexity of the device and most of the .,plasma may be lost in theprocess.

Thus, it would be highly desirable to. produce theplasma Within a region,netic field within the chamber. An axial electrode encircled by acoaxial metallic sleeve projects for a short distance into one end ofthe chamber. A charge of deuterium is admitted to the chamber and alocal radial electrical discharge is created from the central electrodeto the sleeve by applying a high voltage pulse therebetween from atriggered power supply. The radial current, in

conjunction with the axial magnetic field, causes rotation of the plasmacreated by the discharge. A hydromagnetic ionization wave then proceedsfrom the electrode along the length of the chamber, following themagnetic field, leaving behind a gas that is practically fully ionized.The moving ionizing front, or transition region between the plasma andthe neutral gas, is relatively thin and well defined.

Just before the hydromagnetic ionization front reaches the opposite endof the chamber, the inputpulse line is short circuited or crowbarred tohalt the ionizing wave before it contacts the end wall so thatimpurities will not be released therefrom. The plasma rotation stops, much as-short-circuiting a freely spinning electric motor brakes it to astop. The chamber is therefore left filled with a nearly fully ionizedgas. Owing to the described novel structure this plasma has been createdby remotely located electrodes and is therefore relatively free fromcontaminants.

The magnetic field intensity may then be increased to compress and heattheplasma and the configuration of the field may be altered to provideadditional trapping eificiency. The heated plasma may be then utilizedfor various purposes, for instance, ions may be extracted, free neutronsmay be produced, and fusion reactions may be initiated through processeswell known in the art. Typical processes of this type are described, forexample, in the text: Controlled Thermonuclear Reactions, by

,where it is surrounded by a containing magnetic field V 3,14,345Patented Sept. 17, 1963 Glasstone and Lovberg, D. Van Nostrand and Co.,Inc, 1960, pages 6-44.

It is accordingly'an object of the present invention to provide animproved means for creating and heating an electrical plasma.

It is a further object of the present invention to provide a means forreadily obtaining a highly ionized plasma. 7

It is a further object of the present invention to provide a means forcreating a plasma at a point remote from electrodes and other structure.a

It is still a further object ofv the present invention to provide ameans for generating and heating a plasma with minimized introduction ofimprities into the plasma.

It is yet another object of the present invention to provide a simpleand eflicient means for producing a highly ionized plasma within acontaining magnetic field by the initiation of an ionizing wave in a gasdisposed within said fiield.

The invention together with further objects and advantages thereof willbe better understood by reference to the accompanying drawing of which:

FIGURE 1 is a broken out longitudinal view of a plasma generating andcontaining device with certain associated components being shownschematically, and

FIGURE 2 is a longitudinal section view showing an element of the deviceof FIGURE 1 as modified for low gas pressure operation.

Referring now to the drawing and more particularly to FIGURE -1 thereof,there is shown an outer cylindrical shell 5 made of stainless steel or asimilar conductive material and having in this instance a lengthconsiderably exceeding the diameter. First and secondflat circular endclosures 7 and '8 are at opposite ends of the shell 6, forming theplasma chamber 9. Closure 7 is comprised of a conductive material andhas a disc-like configuration with a central aperture 1 3. An electrode'11 of molybdenum or similar material is disposed axially within theaperture 13 and is encircled by a spaced apart coaxial cylindricalconductive sleeve 12, the electrode 11 being supported by an annularinsulator I14 disposed between the sleeve and electrode. The sleeve 12protrudes for a short distance into the chamber 9, extending well beyondthe electrode I l. The outer end 15' of the electrode 11 is threaded andpasses through a circular insulator :14 made of a material such asquartz or alumina ceramic. The electrode 11 is secured to the insulator14 by a nut '16 engaged on the threaded stem portion 15 of theelectrode.

The second end closure 8 of shell 6 may be made of either a conductivematerial or an insulative material, there being openings therein forconnections to both a vacuum pump 17 which removes substantially all theatmosphere in the chamber 9 and a gas source 18 which supplies anioniza-ble gas such as hydrogen, deuterium, or tritium to the chamber 9through a control valve 20. Conventional O-ring vacuum seals 19 aredisposed between the various elements Where necessary to maintain thevacuum.

To provide a containment field, an annular center magnet coil 21 isdisposed coaxially around the outside of the shell 6. A first end coil22 is disposed around the electrode 11 end of shell 6 and a second endcoil 23 is disposed at the opposite side of the central magnet coil 21,the three coils being coaxial. Coils 22 and 23 are adapted to create amore intense magnetic field than the central coil 21, thereby formingthe well known magnetic mirror type field. End coil 22 is made longerthan coil 23 so that in the region within the sleeve 12 the magneticfield is substantially parallel to the axis of the apparatus. A powersupply 24 is connected to the central coil 21 and to the magnetic mirrorend coils 22 and 23 for supplying current thereto to establish the fieldwithin the chamber 9, the configuration of which field is indicated bydashed lines 25.

To generate the plasma it is required that a high energy pulse beapplied to the electrode '11, creating a discharge from the centralelectrode 11 to the sleeve 12, which is at ground potential. Accordinglya high voltage power supply 26 is connected through a current limitingresistor 27 to a pulse line 28 comprised of a plurality of inductors 29and capacitors 31 arranged as a low pass filter. The capacitors 31 arecharged to the full potential of the power supply 26-, -a positivepotential being shown in FIGURE 1, although a negative potential is alsosuitable. A firing ignitron 32 has an anode 33 connected to the pulseline 28. A trigger electrode 36 in the ignitron 32 is connected to theoutput of a trigger pulse generator 37 and receives turn-on pulsestherefrom which initiates conduction between a mercury-pool cathode 38and the anode 33. The cathode 38 is connected to the electrode 11. Whenthe ignit-ron 32 is fired, a potential difference is created between theelectrode 11 and the grounded sleeve 12 'and causes an ionizingwavefront to progress axially through the chamber 9. The pulse line 28provides an approximately constant current over the time period it isutilized.

For reasons previously discussed, it is desirable to stop the wavefrontbefore it strikes the second end closure 8 at the opposite end of thechamber 9. For this purpose a shorting or crowbar ignitron 39 has ananode 41 connected to the electrode 11 and has a mercury-pool cathode 42connected to ground potential. The ionizing wavefront will reach thesecond end closure 8 at a definite interval after the firing ignitron 32starts to conduct, such interval being a characteristic of theparameters of each particular embodiment and being readily determinedempirically. Therefore a portion of the triggering pulse output of thetrigger pulse generator 37 is passed through a delay circuit 43 for suchinterval. The output of such delay circuit is connected to a triggerelectrode 44 in the shorting ignitron 39 and initiates conductiontherethrough.

Considering now the operation of the invention, assume that the variousoperating potentials are applied, that the magnetic field coils 21, 2'2and 23 are energized, and

that the vacuum system and gas sources are operative. The pulse line 28is fully charged until the generation of a pulse from the trigger pulsegenerator 37, whereupon the firing ignitron 32 commences to conduct andthe po- Itential from the pulse line 28 is applied between the electrode11 and the sleeve 12. A local breakdown of the gas in the chamber 9occurs, creating plasma, and a radial current flows from the sleeve 12to the electrode 11. This radial current, together with the axialmagnetic field 25, exerts an azimuthal force on the plasma which causesit to rotate and thus develop a hack electromotive force which tends toreduce the radial current flow. The ionization proceeds along the lengthof the chamber 9, following the field lines 25 so that the gas within acentral axial volume of the chamber is ionized. The width of thetransition region or wavefront between the warm highly ionized rotatingplasma and the essentially neutral gas may be in the order of a fewcentimeters. The process is referred to as a switch-on ionizing wave,because as the wave front passes by, an azimuthal component of magneticfield is switched-on.

If the ionizing current continues to flow after the wavefront hasreached the far end of the chamber 9, deleterious impurity elements fromthe second end plate 8 appear in the plasma. Therefore the potentialapplied to the electrode 11 is shorted out, or crowbarred, by theshorting ignitron 42 just as the ionizing wave reaches the end of thechamber 8. This stops the energy flow from the pulse line 28 to thechamber 9, and also abruptly stops the rotation of the plasma in thesame way that a rotating motor or generator will abruptly stop if thearmature is shorted out.

The gas in the central volume of the chamber 9 is nearly completelyionized by the above process, creating an essentially pure plasma whichis entrapped within the magnetic mirror field 25. The plasma has beenheated in the foregoing process of formation and further heating may beobtained if desired by increasing the intensity of the magnetic field orby other means well known in the art.

Under some conditions, especially with a low gas pressure in the chamber9, breakdown of the gas is facilitated by providing a spark gap at thecenter of the electrode 11 as shown in FIGURE 2.

Referring now to FIGURE 2 in conjunction with FIG- URE 1, there is shownan enlarged modified view of the electrode 11' including the threadedstem 15', the modified electrode having an axial bore 51 therethrough. Asingle conductor 52 is disposed along the axis of the bore 51 and iselectrically isolated from the electrode 11' by an insulator 53. A step54 on the insulator 53 and a corresponding shoulder 55 in the bore 51provide means for compressing an O-ring vacuum seal 19 between theinsulator 53 and electrode 11'. The insulator 53 is secured in positionby a nut 56 on the end of threaded portion 515'. At the chamber 9 sideof the electrode 11' the conductor 52 extends beyond the insulator 53 sothat a small gap separates the end of the conductor 52 from theelectrode. A low capacity capacitor 57 is connected from the conductor52 to ground.

In operation, when a high voltage is applied to the electrode 11'through the firing ignitron 32, for an instant the full pulse line 28potential appears across the gap be tween the electrode 11 and theconductor 52. Breakdown occurs .across the gap, providing a copiousquantity of electrons and ions which cause an immediate breakdown fromthe electrode 11 to the sleeve 12. The capacitor 57 quickly charges tothe full potential of the pulse line 28 and the arc between theelectrode 11 and conductor 52 is quickly extinguished. As a furthermodification, the energy for the spark may be supplied from a separatepulsed power supply which is synchronized with the functioning of thetrigger pulse generator 37.

Ions may be extracted from the plasma and particle interactions in theheated plasma create neutrons which may be utilized for various purposessuch as the irradiation of materials. As the degree of plasma heating isincreased, fusion type reactions occur at an increasing rate throughparticle interactions which are also Well known within the art.

In one embodiment of the invention the chamber 9 is 86 centimeters longand 20 centimeters in diameter. Typical magnetic mirror field intensityis approximately 15 kilogauss requiring a total magnetic energy of 150kilogauss. The pulse line 28 supplies an output potential of 10kilovolts from ten 7.5 microfarad capacitors. The chamber 9 is filledwith hydrogen gas at a pressure of 0.1 millimeter mercury and the degreeof ionization exceeds 90 percent. The velocity of the ionizing wavefrontis approximately 5 centimeters per microsecond, thus the shortingignitron 39 is triggered approximately 15 microseconds after the firingignitron 32 is triggered. The two ignitrons are both RCA type 5550.

The apparatus may be operated in various ways with regard to the mannerin which the magnetic fields of coils 21, 2'2 and 23 are energized. Thecurrent from the magnet current power supply 24 may be adjusted toprovide an axial magnetic field within the chamber 9 during theionization process after which the current through coils 22 and 23 isincreased to provide magnetic mirror fields at each end of the chamber 9to suppress escape of the plasma.

It will therefore be apparent to those skilled in the art that manyvariations and modifications are possible without departing from thespirit and scope of the invention and thus it is not intended to limitthe invention except as defined in the following claims.

What is claimed is:

1. In a magnetohydrodynamic device ionizing a gas by means of anionizing wave, the combination comprising a cylindrical shell forming avacuum chamber, means producing a longitudinally directed magnetic fieldin said chamber which field has increased intensity at each end of aplasma region within said chamber thereby forming a magnetic mirrorfield at each end of said region, a pair of spaced coaxial electrodesdisposed at one end of said region within said mirror field thereat, ahigh volt-age power supply, a first switch coupling said power supply toa first of said electrodes, at second switch connected to provide ashort circuit between said electrodes, control means closing said secondswitch an interval after said first switch closes, and a gas supplycommunicating with said chamber.

2. In a magnetohydrodynamic device as described in claim 1, the furthercombination comprising a spark initiating means disposed in theinnermost of said electrodes.

3. A magnetohydrodynamic device as described in claim 1 furthercharacterized by said control means having a timing element closing saidsecond switch after an interval substantially equal to the transit timeof the wavefront of said ionizing wave through said plasma region.

4. In a magnetohydrodynamic device, the combination comprising anannular magnetic field coil having spaced end sections each providing astrong mirror field and an intermediate section providing a field ofless intensity which forms a plasma trapping region, a first electrodedisposed within the field of a first of said end sections along .theaxis thereof and being confined to a position outside said plasmatrapping region, a second sleeve electrode disposed within the field ofsaid first end section in coaxial relationship with said first electrodeand radially spaced therefrom to form a discharge gap, a first switch, ahigh voltage power supply coupled to said first and second electrodesthrough said first switch for applying a potential difierence to saidelectrodes, a second switch connected between said first and secondelectrodes, control means closing said second switch an interval aftersaid first switch closes, a vacuum vessel enclosing said plasma trappingregion, and means supplying gas to said vacuum vessel.

5. In a device for generating a highly ionized plasma, the combinationcomprising a cylindrical shell defining a vacuum chamber, a firstmagnetic field coil disposed coaxially around :a central portion of saidshell, a second and third magnetic field coil disposed coaxially aroundsaid shell one at each end of said first coil, said second and thirdcoils providing more intense magnetic fields than said first coilwhereby a plasma trapping region is established within said controlportion of said shell, a first electrode disposed along the axis of saidchamber and being limited to a longitudinal position thereincorresponding to that of said second field coil, a second sleeveelectrode disposed coaxially around said first electrode and beingspaced radially from said first electrode and from said shell, a highvoltage source, a first switch connected between said high voltagesource and said first and second electrodes for applying a potentialdiiference therebetween, a second switch connected from said first electrode to said second electrode, a switch control circuit connected tosaid first switch and said second switch, said control circuit closingsaid second switch a fixed interval after said first switch is closed,and a gas source communicated with said chamber. 7

6. A device as described in claim further charac- 6 terized by a sparkinitiating means disposed at said first electrode.

7. In a plasma generating and containment device, the combinationcomprising a long vacuum vessel, a magnetic field coil disposed aroundsaid vessel and adapted to provide a substantially longitudinal magneticfield therethrough which field includes a central plasma trappingregion, an electrode disposed substantially at the axis of said vesseladjacent one end of said plasma trapping re gion, a sleeve coaxial withsaid electrode and extending beyond said electrode towards said plasmatrapping region, a high voltage power supply, a pulse line connected tothe output of said high voltage power supply, a first switch connectedfrom said pulse line to said electrode, a second switch connected fromsaid electrode to said sleeve, a switch control circuit connected tosaid first and second switches and closing said second switch aninterval after the closure of said first switch, a vacuum pump coupledto said vessel, and a gas source coupled to said vessel.

8. A plasma generating and containment device as de scribed in claim 7,wherein said first and second switches are ignitrons, and a delaycircuit connected from said control circuit to said second switch.

- 9. In a plasma heating and containment devicepthe combinationcomprising a. cylindrical vacuum enclosure, a magnet coil disposedcoaxially anound said enclosure and having end sectors providing amagnetic field of higher intensity than a central sector, a magnetcurrent power supply coupled to said magnet coil, an electrode disposedalong the axis of said enclosure at a longitudinal positioncorresponding to that of a first of said end sectors of said coil, anannular sleeve disposed coaxially around said electrode and beingradially spaced from said electrode and said enclosure, a high voltagepower supply connected trom said electrode to said sleeve, a firstignitron connecting said high voltage power supply to said electrode,said first ignitron having a first trigger electrode, a pulse generatorconnected to said first trigger electrode, a second ignitron connectedfrom said electrode to said sleeve and having a second triggerelectrode, a delay circuit connected from said pulse generator to saidsecond trigger electrode, a vacuum pump coupled to said enclosure, and agas supply coupled to said enclosure.

10. In a gas ionizing device, the combination comprising a firstcylindrical electrode, a second cylindrical electrode disposed coaxiallyaround said first electrode and spaced apart therefrom, an electricalpower supply connected across said first and said second electrodes,first switch means connected between said power supply and one of saidelectrodes, magnetic field producing means providing an axially directedmagnetic field in the zone between said first and said second electrodeswhich field extends a substantial distance past said electrodesproviding an unobstructed plasma containment region adjacent one end ofsaid electrodes, a vacuum tank enclosing said first and said secondelectrodes and said containment region, a gas source communicableWith'said tank, a second switch means connected between said firstelectrode and said second electrode, and a control circuit closing saidsecond switch an interval after said first switch closes.

References Cited in the file of this patent UNITED STATES PATENTS2,992,345 Hansen Julyrll, 1961 3,005,931 Dandl Oct. 24, 1961 3,048,736Emmerich Aug. 7, 1962 3,064,178 Persson Nov. 13, 1962

1. IN A MAGNETOHYDRODYNAMIC DEVICE IONIZING A GAS BY MEANS OF ANIONIZING WAVE, THE COMBINATION COMPRISING A CYLINDRICAL SHELL FORMING AVACUUM CHAMBER, MEANS PRODUCING A LONGITUDINALLY DIRECTED MAGNETIC FIELDIN SAID CHAMBER WHICH FIELD HAS INCREASED INTENSITY AT EACH END OF APLASMA REGION WITHIN SAID CHAMBER THEREBY FORMING A MAGNETIC MIRRORFIELD AT EACH END OF SAID REGION, A PAIR OF SPACED COAXIAL ELECTRODESDISPOSED AT ONE END OF SAID REGION WITHIN SAID MIRROR FIELD THEREAT, AHIGH VOLTAGE POWER SUPPLY, A FIRST SWITCH COUPLING SAID POWER SUPPLY TOA FIRST OF SAID ELECTRODES, A SECOND SWITCH CONNECTED TO PROVIDE A SHORTCIRCUIT BETWEEN SAID ELECTRODES, CONTROL MEANS CLOSING SAID SECONDSWITCH AN INTERVAL AFTER SAID FIRST SWITCH CLOSES, AND A GAS SUPPLYCOMMUNICATING WITH SAID CHAMBER.